1. Field of the Invention
This invention relates to a micromachined probe apparatus and methods for making and using same to characterize liquid in a fluidic channel and map embedded charge in a sample on a substrate.
2. Background Art
U.S. Pat. Nos. 6,624,377 and 6,586,699 are related to the present application.
The following references are referred to in this specification:
[And52]J. R. Anderson et al., “Theory of the Vibrating CondenserConverter and Application to Contact PotentialMeasurements,” AUSTRALIAN JOURNAL OF APPLIED SCIENCE, 3, 201,1952.[Ber95]P. L. Bergstrom et al., “Dielectric Membrane Technology forConductivity and Work-Function Gas Sensors,” IEEEINTERNATIONAL CONFERENCE ON SENSORS AND ACTUATORS(Transducers, 1995), Stockholm, Sweden, pp. 993–996,1995.[But92]H. -J. Butt, “Measuring Local Surface Charge Densities inElectrolyte Solutions with a Scanning Force Microscope,”BIOPHYSICAL JOURNAL, 63(2), pp. 578–582, 1992.[Chu03]L. L. Chu et al., “A Micromachined Kelvin Probe for SurfacePotential Measurements in Microfluidic Channels and Solid-State Applications,” IEEE INTERNATIONAL CONFERENCE ONSENSORS AND ACTUATORS (Transducers, 2003), Boston,USA, pp. 384–387, 2003.[Cis98]C. Cismaru et al., “Relationship Between the ChargingDamage of Test Structures and the Deposited Charge onUnpatterned Wafers Exposed to an Electron CyclotronResonance Plasma,” APPL. PHYS. LETT., 72(10), pp. 1143–1145,1998.[Fan92]S. Fang et al., “Thin-Oxide Damage from Gate ChargingDuring Plasma Processing,” IEEE ELECTRON DEV. LETT.,13, p. 288, 1992.[Fri97]J. B. Friedmann et al., “Plasma-Parameter Dependency ofThin-Oxide Damage from Wafer Charging During Electron-Cyclotron-Resonance Plasma Processing,” IEEE TRANS.SEMIC. MFG., 10(1), 154, 1997.[Hei99]W. F. Heinz et al., “Relative Surface Charge DensityMapping with the Atomic Force Microscope,” BIOPHYSICALJ., 76, pp. 528–538, 1999.[Hof97]A. Hoff et al., “A Novel Approach to Monitoring of PlasmaProcessing Equipment and Plasma Damage Without TestStructures,” IEEE/SEMI ADVANCED SEMIC.MANUFACTURING CONF., 185, 1997.[Hun93]Robert J. Hunter, “Introduction to Modern Colloid Science,”OXFORD SCIENCE PUBLICATIONS, 1993.[Lud01]R. Ludeke et al., “Imaging of Oxide and Interface Chargesin SiO2-Si,” MICROELECTRONIC ENGINEERING, 59, pp. 259–263,2001.[Man97]F. Man et al., “Microfluidic Plastic Capillaries on SiliconSubstrates: A New Inexpensive Technology for BioanalysisChips,” IEEE MEMS, Nagoya, Japan, pp. 311–316, 1997.[Mor00]S. Morita et al., “Defects and their Charge Imaging onSemiconductor Surfaces by Non-Contact Atomic ForceMicroscopy and Spectroscopy,” J. CRYSTAL GRTH., 210, pp.408–415, 2000.[Mur73]P. V. Murphy et al., “Blood Compatibility of PolymerElectrets,” PROC. INT. CONF. ON ELECTRETS, CHARGESTORAGE, AND TRANSPORT IN DIELECTRICS, Miami Beach,FL, Electrochemical Society, Princeton, NJ, pp. 627–649,1973.[Nab97]W. Nabhan et al., “A High-Resolution Scanning KelvinProbe Microscope for Contact Potential Measurements on the100 nm Scale,” REVIEW OF SCIENTIFIC INSTRUMENTS, 68(8),p. 3108, 1997.[Pet99]I. R. Peterson, “Kelvin Probe Liquid-Surface PotentialSensor,” REVIEW OF SCIENTIFIC INSTRUMENTS, 70(8), pp.3418–3424, Aug. 1999.[Que01]L. Que et al., “Bent-Beam Electro-thermal Actuators-I:Single Beam and Cascaded Devices,” J. MICROELECTROMECH.SYS., 10(2), pp. 247–254, 2001.[Rai96]R. Raiteri et al., “Measuring Electrostatic Double-LayerForces at High Surface Potentials with the Atomic ForceMicroscope,” JOURNAL OF PHYSICAL CHEMISTRY, 100, pp.16700–16705, 1996.[Sur70]N. A. Surplice et al., “A Critique of the Kelvin Method ofMeasuring Work Functions,” J. PHYSICS E: SCIENTIF.INSTRUM. 3, 477, 1970.[Tak02]K. Takahata et al., “Batch Mode Micro-electro-dischargeMachining,” J. MICROELECTROMECH. SYS., 11(2), pp. 102–110,2002.
The vibrating Kelvin probe is an effective, non-invasive tool for the non-contact mapping of surface potentials [And52, Sur70, Nab97]. Since surface potential includes a component due to work function and another due to trapped charge, this tool can be used to map either quantity on a surface when the other is kept uniform. For example, trapped charge is monitored in semiconductor IC-fabrication because it has been correlated to the degradation of the device parameters. This function can be performed by mapping the contact (or surface) potential difference (CPD) between a probe and the sample wafer [Hof97]. Since the Kelvin probe method is a non-contact and non-destructive diagnostic, it can be used to monitor processes that are known to introduce trapped charges in wafers, such as plasma etch and deposition, ion implantation, and certain cleaning and wafer drying operations. Conventional plasma damage characterization approaches [Fri97, Cis98, Fan92] are based on electrical or surface analytical techniques, which cannot measure local charge distributions on patterned wafers. The ability to measure local charge distributed across patterns on production wafers can be a critical asset in predicting yield and longevity of devices.
Another potential application for scanning Kelvin probes is the mapping of surface charge in a biofluidic channel or tube [Bai98]. Charge distribution on the wall, acquired during manufacturing or in routine operation, can impact the function and behavior of the fluid in the channel. For example, it has been linked to cell adhesion and clotting of red blood cells on artificial surfaces [Mur73]. A Kelvin probe can be used to map the charge embedded in the wall by scanning the outside of the fluidic channel using the electrolyte in the channel as an electrode. A macro-scale Kelvin probe has also been used to measure the surface potentials of organic overlayers on poorly conducting liquid substrates [Pet99], and a micromachined device based on this principle has been developed for gas sensing [Ber95].
Relative surface charge density has been measured suing atomic force microscopy (AFM) in an aqueous electrolytic ambient [But92, Rai96, Hei99]. When the AFM tip is in close proximity to the sample, the electrostatic force produced by their overlapping electrical double layers is detected by the AFM tip and correlated to charge density. This method is effective for measurements of biological samples in an aqueous environment. In non-aqueous environments, non-contact AFM methods have recently been used for the mapping of surface [Mor00] and interface [Lud01] charge on a semiconductor. However, these experiments require ultra high vacuum (UHV) and, in some cases, cooling of the system to liquid He temperature, which limit their application.